The present invention relates to super heterodyne receivers, especially those which use multiple stages of mixers and filters.
FIG. 1 illustrates a prior art dual stage super heterodyne receiver 20. Signals are detected by the antennae 22 and filtered in band pass filter 23. Low noise amplifier 25 amplifies the signal and sends it to the first image filter 28. The mixer 24 receives the filtered signal and a signal from the local oscillator 26. The output of the mixer is sent to second image filter 34 which ideally filters away the undesired second image signals. A second stage also includes mixer 30.
A problem that can exist with super heterodyne receivers is that the filter 34 may not be able to completely remove the undesired image signal. It is desired to have an improved system that can more efficiently remove any second image frequencies.
It has been found that in small form factor systems, such as those used with cellular radio systems, undesired energy can bypass an image filter in the heterodyne receiver by magnetically coupling to and from a metallic radio frequency (RF) shield positioned close to the filter. The signals at the input of the filter magnetically couple to the RF shield, conduct along the surface of the shield and then couple from the shield to the output of the filter. This is especially a problem with a high impedance filters such as the type typically found in the first intermediate frequency (IF) stage of a dual-stage superheterodyne receiver. Such a high impedance filter typically uses large inductors in the matching networks at the input and output of the filter. Energy radiated by the large inductors can bypass the filter through the metallic RF shield.
Increasing the physical separation between the metal RF shield and the filter is one possible solution to this problem. However, with the increasing pressure to reduce the size of electronic devices, this solution is becoming less and less attractive. Another way to prevent this problem is by using complicated shielding structure in an attempt to isolate the input and output of the intermediate frequency filter. This solution is costly and difficult to manufacture.
The present invention comprises the use of a ferrite embedded element between the filter and the metallic RF shield to dissipate the undesired energy which radiate at the input of the filter and prevent such energies from bypassing the filter through the radio frequency shield. The ferrite embedded element will dissipate the radiated electromagnetic energy into heat. By using such a ferrite embedded element, the radio frequency shielding can be positioned close to the filter and the small form factor of the receiver maintained.